The use of non-aqueous dispersions (NADs) has enabled the development of fast setting sheet fed inks. However, due to either residual monomers or fractions of initiators, these NADs often have an objectionable odor.
As is known, a residual monomer is an un-polymerized monomer that remains incorporated in a polymer after the polymerization reaction has completed. Many residual monomers are malodorous, and some are even toxic. For example, acrylic acid and methacrylic acid, as well as peroxides used as initiators, have objectionable odors.
Aqueous emulsion polymerization systems are well known for the production of many types of polymers, including, for example, rubbery styrene-butadiene polymers as well as polymers containing acrylonitrile (“ACN”). In commercial processes, it is desirable to minimize the amount of residual monomer in a latex, for various reasons. From an environmental point of view so as to minimize exposure to hydrocarbon monomers which may be odiferous or may present more serious health concerns; from an economic point of view eliminating the last of 1.0 to 0.5 percent of free monomer is a significantly costly step, and from an efficiency point of view inasmuch as the current methods of reducing the residual monomer in latex tend to be time consuming and slow production down. Furthermore, these processes tend to be energy intensive, thus adding additional costs. More over the novel process allows a manufacturer to avoid having to deal environmental regulations regarding residual monomers.
S. M. Englund, in his paper Monomer Removal from Latex, Chemical Engineering Progress (August 1981), p. 55, discusses some of the problems the industry faces. In latex production, the removal of residual hydrocarbon monomer is limited by the rate at which the latex can be contacted with a fresh stream of a fluid, usually steam, to remove the monomer. Thus, the latex may need to be contacted with relatively high amounts of steam to ensure the adequate removal of residual monomer, which can lead to various types of problems such as, for example, foaming, latex destabilization or latex degradation.
The B. F. Goodrich Company has developed a number of methods to remove residual acrylonitrile (“ACN”) from aqueous polymeric emulsions. For example, U.S. Pat. No. 4,251,412 discloses a process for the reduction of residual ACN in a latex by adding a stoichiometric amount of amine to react with the CAN; U.S. Pat. No. 4,272,425 discloses a process for reducing the ACN content in latex by adding additional catalyst and co-monomer in excess of the free ACN after 90 percent conversion of the monomers; and U.S. Pat. No. 4,278,582 discloses a process for reducing the residual ACN content of latex by adding ammonia, ammonium hydroxide, thiols, hydrogen sulfide and its salts and alkali and alkaline earth metal carbonates, phosphates and silicates.
Other known methods for reducing residual monomers contemplate fluid injection, usually steam or an inert gas, and/or passing the latex through a reactor at reduced pressure.
Generally in emulsion polymerization, the polymerization conversion rate rapidly increases during an early induction period. However, once the polymerization reaction reaches the point of 94% conversion, the polymerization conversion rate decreases. This phenomenon is especially conspicuous in rubber reinforced latex, where the monomer content drops rapidly once the polymerization conversion rate reaches 94%. This is believed to be caused by the interruption of an initiator (such as, for example, a peroxide) for the permeation of monomers into a rubber particle because of lack of diffusivity. Additionally, a shorter reaction time can likely result in a decrease of the polymerization conversion rate. In the meantime, high rubber content and high solid content makes the migration of monomers into the inside of rubber particles easy, thus also causing a decrease in the polymerization conversion rate.
Methods have also been proposed to reduce residual monomers during polymerization. Thus, German Patent No. 19741188 describes a method that uses a long-chained initiator for the polymerization, WO2000/014123 describes a method to treat alpha-hydroxy carbonyl compound with an antioxidant, U.S. Pat. No. 6,620,883 introduces multiple initiator treatment, and WO2003/010214 describes a method including the additional insertion of a third monomer such as acrylate. Methods to increase drying efficiency after the polymerization process, or to increase the vacuum during extruding and injecting, have also been suggested.
However, there is a limit to the ability of these various methods to eliminate residual monomers. Furthermore, (i) the physical properties of the resin itself could be changed using these methods, such as, for example, their molecular weight—inasmuch as the implementation of these methods generally causes additional polymerization to occur, or (ii) productivity can be decreased by a longer reaction time.
Moreover, none of the prior art refers to techniques for reducing odor caused by residual monomers or initiator fractions or side reactions in non aqueous dispersions (NADs). This is because the introduction of large quantities of water (such as, for example, via steam injection, as described above) destabilizes the NADs.
What is thus needed in the art is a faster and less energy consuming method of reducing the residual monomer content in lattices. What is further needed is such a method that will additionally not change the physical properties of the resin or reaction product, or increase reaction time.